1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus, and more particularly relates to a switching power supply which reduces a sound in an audible range generated by a transformer during light load operation.
2. Description of the Related Art
FIG. 5A illustrates an example of a switching power supply. When a load becomes light, the switching power supply is controlled so that a switching element is driven in an intermittent oscillation state to reduce power consumption. FIG. 5A will be described later in detail in Description of the Embodiments. It is known that a sound is generated from the transformer when a cycle or a high frequency component of intermittent oscillation of the switching power supply enters into an audible range. While the switching power supply of an electronic apparatus operates in the intermittent oscillation state, a dominant load mode (load state) of the electronic apparatus is a sleep mode. In a stand-by mode, the intermittent oscillation state is supported in a range which is very low as the load state (which is close to the sleep mode). FIG. 5B is a concept view illustrating the state of the switching power supply corresponding to the load mode of the electronic apparatus. The load mode of the electronic apparatus includes, for example, an operation mode, a stand-by mode and a sleep mode. The state of the switching power supply includes a continuous oscillation state and an intermittent oscillation state. In the operation mode, the electronic apparatus is executing operation, while in the stand-by mode, the electronic apparatus is waiting for an operation instruction without executing any operation (is ready for executing operation). In the sleep mode, the electronic apparatus is in an energy saving state in which supply of electric power is partially stopped. (A) in FIG. 5B is a concept view illustrating the state of a conventional switching power supply, in which a ratio (α) of the sleep mode is considerably larger than a ratio (β) of the stand-by mode in the intermittent oscillation state. Accordingly, as a countermeasure against sound generation by the transformer during light load operation, a signal indicative of the sleep mode is transmitted from a control unit of the electronic apparatus to the switching power supply. By using the signal indicative of the sleep mode as a trigger, the switching element is controlled so as to have a turn-on width forcibly narrowed (see, for example, Japanese Patent Application Laid-Open No. 2000-184709).
However, in recent years, power consumption of electronic apparatuses becomes much lower than before. Under the influence of the lowered power consumption, the stand-by mode becomes more dominant than the sleep mode as the load mode that supports the intermittent oscillation state of the switching power supply. Accordingly, sound generation by the transformer is more noticeable in the stand-by mode than in the sleep mode. (B) in FIG. 5B is a concept view illustrating the state of a recent switching power supply, in which a ratio (γ) of the sleep mode is considerably smaller than a ratio (δ) of the stand-by mode in the intermittent oscillation state. In the past, the control unit of the electronic apparatus has performed a countermeasure against the sound generation by the transformer by using as a trigger the signal indicative of the sleep mode. However, it becomes more and more difficult to successfully perform the countermeasure. To cope with this situation, the countermeasure against sound generation may be performed on the entire load. However, this involves narrowing a turn-on width (turn-on time) of the switching element even in the operation mode that handles a heavy load. As a result, a driving frequency is increased even in the operation mode that handles the heavy load, which hinders reduction in power consumption and causes increase in switching noise.